def p(n):
    for i in range(2, n)
    if n % i==0:
        return False
    return True

x=int(input())

prost = [i for i in range(1, 10000) if p(i)]


m=0
if 1000<=x<=9990:
    for i in prost:
        if str(i) on str(x):
            m = max(i, m)
print(m)
    
        
